arXiv subjects

Heather E. Logan

Publications and source records attributed to Heather E. Logan.

At least 19 recordsLinked to original sources

arXiv2026

Analytic Approximations for Fermionic Preheating

Non-thermal fermions can be produced non-perturbatively in the early universe during coherent oscillations of a scalar field. We explore fermion production in $\lambda\phi^{4}$ inflation through this mechanism and analyze the momentum spectrum of the fermions produced, which depends on a coupling parameter $q$. For $q \gtrsim 0.01$, the main contribution to the total number density comes from an approximately half-filled Fermi sphere as a result of non-adiabaticity. For $q\lesssim 0.01$, we find that the major contributions instead come from resonance peaks at higher momentum values. We find a simple relation to predict the momentum values corresponding to resonance peaks for any $q$. We also obtain analytic power-law approximations for the total number density of fermions and find that it is proportional to $q^{1/2}$ for $q\lesssim 0.01$ and proportional to $q^{3/4}$ for $q\gtrsim 10$. If fermions produced by this mechanism make up the entirety of dark matter, we estimate lower bounds on their mass.

hep-ph↗

arXiv2025

Lectures on perturbative unitarity and decoupling in Higgs physics

These lectures are a pedagogical introduction to the application of perturbative unitarity to Higgs physics within and beyond the Standard Model (SM). I begin with a review of how perturbative unitarity arises from quantum mechanical scattering theory and apply it to the classic problem of longitudinal vector boson scattering in the SM to derive the famous upper bound on the Higgs boson mass. I then consider extended Higgs sectors, using the two-Higgs-doublet model, the scalar septet model, and the Georgi-Machacek model as case studies. I discuss the resulting Higgs coupling sum rules as well as the intertwined constraints on masses and couplings of the additional Higgs bosons in these models, highlighting the connection between perturbative unitarity and the decoupling limit. I finish with a direct review of the decoupling limit in the two-Higgs-doublet model and a digression on the alignment limit. These notes are based on lectures delivered over the past few years in a graduate-level course on beyond-the-SM phenomenology and are meant as a companion volume to my TASI 2013 lectures on Higgs physics.

hep-ph↗

arXiv2024

How to make your research group more inclusive for autistic trainees

As a university professor, one of my most important responsibilities is mentoring the junior members of my research group and creating an inclusive environment in which they can thrive. Since my autism diagnosis two years ago, colleagues have asked me how they can make their research groups more welcoming to autistic trainees. This short guide, based on conversations with autistic students and academics, intense reflection on my own lived experience, and a deep dive into the literature, provides five concrete steps toward this goal.

physics.ed-ph↗

arXiv2024

Is the real two-Higgs-doublet model consistent?

We provide strong evidence that the widely-studied real two-Higgs-doublet model is inconsistent under renormalization due to quark-induced divergent CP violation (CPV). We identify the necessary ingredients for the CPV to enter the renormalization-group equations based on symmetry proprieties of the divergent diagrams. We demonstrate that while these ingredients are present starting at six loops, the divergent CPV is zero at that order due to approximate symmetries. We show that these symmetries are broken at seven loops and determine the parameter dependence of the resulting divergent CPV.

hep-ph↗

arXiv2024

Can CP be conserved in the two-Higgs-doublet model?

We study the conditions under which the CP violation in the quark mixing matrix can leak into the scalar potential of the real two-Higgs-doublet model (2HDM) via divergent radiative corrections, thereby spoiling the renormalizability of the model. We show that any contributing diagram must involve 12 Yukawa-coupling insertions and a factor of the $U(1)_{PQ}$-breaking scalar potential parameter $λ_5$, thereby requiring at least six loops; this also implies that the 2HDM with only softly-broken $U(1)_{PQ}$ is safe from divergent leaks of CP violation to all orders. We demonstrate that additional symmetries of the six-loop diagrams in the type I and II 2HDMs guarantee that all of the divergent CP-violating contributions cancel at this order. We also show that these symmetries are violated at seven loops and enumerate the classes of diagrams that can contribute to CP-violating divergences, providing evidence that the real 2HDM is theoretically inconsistent starting at the seven-loop level.

hep-ph↗

arXiv2022

Unavoidable Higgs coupling deviations in the $Z_2$-symmetric Georgi-Machacek model

The $Z_{2}$-symmetric version of the Georgi-Machacek model does not possess a decoupling limit in which all the new particles can be made arbitrarily heavy, opening the possibility that the model can be entirely excluded if experiments reveal no deviations from the Standard Model. We explore this model, focusing on the part of parameter space in which the vacuum expectation value of the triplets, $ν_χ$, is small. In the small-$ν_χ$ limit, the second custodial-singlet scalar field $S$ necessarily becomes very light and can contribute to the total width of the 125 GeV Higgs boson $h$ via $h \to SS$. We show that this process, together with LHC measurements of the $h \to γγ$ rate, entirely excludes masses $m_S < m_h/2$ and thereby severely constrains the parameter space, setting an experimental lower bound $ν_χ\gtrsim 12.5$ GeV on the vacuum expectation value of the triplets. This lower bound makes it impossible to avoid deviations from the Standard Model in the couplings of $h$ to fermion and vector boson pairs. We study the remaining parameter space after imposing constraints from direct searches for the additional Higgs bosons, and show that it is on the edge of being fully excluded at $95\%$ confidence level by LHC measurements of the 125 GeV Higgs boson's couplings. Measurements of these couplings at the future high-luminosity run of the LHC will have sufficient precision to entirely exclude the model at $5σ$ if no deviations from the Standard Model are observed.

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arXiv2022

TASI 2013 lectures on Higgs physics within and beyond the Standard Model

These lectures start with a detailed pedagogical introduction to electroweak symmetry breaking in the Standard Model, including gauge boson and fermion mass generation and the resulting predictions for Higgs boson interactions. I then survey Higgs boson decays and production mechanisms at hadron and $e^+e^-$ colliders. I finish with two case studies of Higgs physics beyond the Standard Model: two-Higgs-doublet models, which I use to illustrate the concept of minimal flavor violation, and models with isospin-triplet scalar(s), which I use to illustrate the concept of custodial symmetry.

hep-ph↗

arXiv2022

CP-Violation in the 3-Higgs Doublet Model: CP-Asymmetries from Charged Higgs Bosons and Electric Dipole Moments

We propose a CP-violating source in the charged Higgs sector of a 3-Higgs Doublet Model (3HDM) that will contribute to three CP-asymmetry observables for low energy processes. In the context of such a 3HDM, a measurement of one of the specific asymmetry observables (i.e., $\bar{B} \to X_{s+d} γ$) could obtain up to 2.5% or more, which would give a 5$σ$ evidence for physics Beyond the SM (BSM) from the full BELLE II data set. In addition to these CP-violating observables, the current measurements of Electron Dipole Moments (EDMs) constrain the available parameter region of the CP-violating 3HDM. In this work, we perform a thorough analysis of the parameter space of it to find the regions that satisfy EDMs, $\bar{B} \to X_s γ$, direct collider searches for charged Higgs bosons as well as theoretical bounds.

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arXiv2021

CP violation from charged Higgs bosons in the three Higgs doublet model

We demonstrate a new type of cancellation of contributions to the electron and neutron electric dipole moments (EDMs) that occurs in three Higgs doublet models (3HDMs) when CP violation appears in the charged Higgs sector. The cancellation becomes exact when the two physical charged Higgs bosons in the model are degenerate in mass. Depending on the model parameters, degeneracies at the 10\% level are however sufficient to evade current bounds on the electron and neutron EDMs. We demonstrate that viable parameter space remains with both charged Higgs bosons lighter than 500 GeV and large CP-violating phases while also satisfying theoretical constraints from perturbativity and experimental ones from $\bar{B} \to X_s γ$ and direct searches.

hep-ph↗

arXiv2021

Probing Dark Sector CP Violation with Electric Dipole Moments and Colliders

We study the experimental constraints on dark sector CP violation that enters the visible sector through a Higgs portal coupling $H^{\dagger} H Z^{\prime}_{μν} \widetilde Z^{\prime μν}$, where the $Z^{\prime}$ is from a new U(1) gauge symmetry which is assumed to couple to lepton number. We compute explicitly the leading two-loop contribution of this effective operator to the electron electric dipole moment (EDM) and show that the resulting constraints are comparable to those from direct $Z^{\prime}$ searches at electron-positron colliders when the effective operator is generated at tree level. We also examine an explicit UV completion for this effective operator that was first introduced to achieve electroweak baryogenesis and show that collider constraints from $B$-factories already exclude viable baryogenesis for $Z^{\prime}$ masses below 10 GeV, and that future electron-positron Higgs factories will exclude viable baryogenesis for $Z^{\prime}$ masses up to the $e^+e^-$ center-of-mass energy if anticipated luminosities are achieved. For higher $Z^{\prime}$ masses, the full viable baryogenesis parameter space lies within six orders of magnitude of the current upper bound on the electron EDM.

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arXiv2021

A benchmark for LHC searches for low-mass custodial fiveplet scalars in the Georgi-Machacek model

The Georgi-Machacek (GM) model is used to motivate and interpret LHC searches for doubly charged scalars decaying to vector bosons pairs. The doubly charged scalars are part of a degenerate fermiophobic custodial fiveplet with states $H_5^{\pm\pm}$, $H_5^\pm$, and $H_5^0$ and common mass $m_5$. The GM model has been extensively studied at the LHC for $m_5 > 200$ GeV but there is a largely unprobed region of parameter space from $120$ GeV $< m_5 < 200$ GeV where light doubly-charged scalars could exist. This region has been neglected by experimental searches due in part to the lack of a benchmark for $m_5 < 200$ GeV. In this paper we propose a new "low-$m_5$" benchmark for the GM model, defined for $m_5 \in (50, 550)$ GeV, and characterize its properties. We apply all existing experimental constraints and summarize the phenomenology of the surviving parameter space. We show that the benchmark populates almost the entirety of the relevant allowed parameter plane for $m_5$ below 200 GeV and satisfies the constraints from the 125 GeV Higgs boson signal strengths. We compute the 125 GeV Higgs boson's couplings to fermion and vector boson pairs and show that they are always enhanced in the benchmark relative to those in the Standard Model. We also compute the most relevant production cross sections for $H_5$ at the LHC, including Drell-Yan production of $H_5$ pairs. The process $pp \to H \to H_5 H_5$ contributes in a small region of parameter space, but is small compared to the Drell-Yan production cross section. Finally we compute the branching ratios of $H_5^0 \to γγ$ and $H_5^{\pm} \to W^{\pm} γ$. The width-to-mass ratio of each of the $H_5$ states is below 1\% over the entire benchmark, so that the narrow-width approximation is well justified.

hep-ph↗

arXiv2020

Vacuum structure of the $\mathbb{Z}_2$ symmetric Georgi-Machacek model

We discuss the vacuum structure of a version of the Georgi-Machecek model with an exact $\mathbb{Z}_2$ symmetry acting on the triplet fields. Besides the usual custodial-symmetric model, with $ρ=1$ at tree-level, a model with a dark matter candidate is also viable. The other phases of the model lead to electric charge breaking, a wrong pattern of electroweak symmetry breaking or to $ρ\neq 1$ at tree-level. We derive conditions to have an absolute minimum in each of the two viable phases, the custodial and the dark matter phases.

hep-ph↗

arXiv2020

Custodial symmetry violation in the Georgi-Machacek model

We study the effects of custodial symmetry violation in the Georgi-Machacek (GM) model. The GM model adds isospin-triplet scalars to the Standard Model in a way that preserves custodial symmetry at tree level; however, this custodial symmetry has long been known to be violated at the one-loop level by hypercharge interactions. We consider the custodial-symmetric GM model to arise at some high scale as a result of an unspecified ultraviolet completion, and quantify the custodial symmetry violation induced as the model is run down to the weak scale. The measured value of the eletroweak rho parameter (along with unitarity considerations) lets us constrain the scale of the ultraviolet completion to lie below tens to hundreds of TeV. Subject to this constraint, we quantify the size of other custodial-symmetry-violating effects at the weak scale, including custodial symmetry violation in the couplings of the 125 GeV Higgs boson to W and Z boson pairs and mixings and mass splittings among the additional Higgs bosons in the theory. We find that these effects are small enough that they are unlikely to be probed by the Large Hadron Collider, but may be detectable at a future $e^+e^-$ collider.

hep-ph↗

arXiv2020

Updated constraints on the Georgi-Machacek model from LHC Run 2

We study the experimental constraints on the Georgi-Machacek model from direct searches for the new Higgs bosons and measurements of the properties of the discovered 125 GeV Higgs boson. We apply these by interfacing the public codes HiggsBounds 5.2.0 and HiggsSignals 2.2.1, which implement a large collection of direct-search constraints on extra Higgs bosons and measurements of the properties of the 125 GeV state to the model calculator GMCALC. We also implement constraints involving searches for doubly-charged Higgs bosons and Drell-Yan production of a neutral Higgs with decays to photon pairs directly into GMCALC; these channels are not included in HiggsBounds but provide important constraints on the model, especially at low mass. We identify new constraints on the model from $H_3^0 \to Z h$ and $H \to hh$ and point out that these channels remain promising. We also determine the allowed range of couplings of the SM-like Higgs boson after all experimental constraints are applied and show that the direct searches for additional Higgs bosons are generally more constraining than the measurements of the 125 GeV Higgs boson couplings. The updated GMCALC code will be released as version 1.5.0.

hep-ph↗

arXiv2020

Implications of the observation of dark matter self-interactions for singlet scalar dark matter

Evidence for dark matter self-interactions has recently been reported based on the observation of a spatial offset between the dark matter halo and the stars in a galaxy in the cluster Abell 3827. Interpreting the offset as due to dark matter self-interactions leads to a cross section measurement of sigma_DM/m ~ (1-1.5) cm^2/g, where m is the mass of the dark matter particle. We use this observation to constrain singlet scalar dark matter coupled to the Standard Model and to two-Higgs-doublet models. We show that the most natural scenario in this class of models is very light dark matter, below about 0.1 GeV, whose relic abundance is set by freeze-in, i.e., by slow production of dark matter in the early universe via extremely tiny interactions with the Higgs boson, never reaching thermal equilibrium. We also show that the dark matter abundance can be established through the usual thermal freeze-out mechanism in the singlet scalar extension of the Yukawa-aligned two-Higgs-doublet model, but that it requires rather severe fine tuning of the singlet scalar mass.

hep-ph↗

arXiv2018

Searching for the $W γ$ decay of a charged Higgs boson

We study the prospects for charged Higgs boson searches in the $W γ$ decay channel. This loop-induced decay channel can be important if the charged Higgs is fermiophobic, particularly when its mass is below the $WZ$ threshold. We identify useful kinematic observables and evaluate the future Large Hadron Collider sensitivity to this channel using the custodial-fiveplet charged Higgs in the Georgi-Machacek model as a fermiophobic benchmark. We show that the LHC with 300~fb$^{-1}$ of data at 14~TeV will be able to exclude charged Higgs masses below about 130~GeV for almost any value of the SU(2)$_L$-triplet vacuum expectation value in the model, and masses up to 200~GeV and beyond when the triplet vacuum expectation value is very small. We describe the signal simulation tools created for this analysis, which have been made publicly available.

hep-ph↗

arXiv2018

Scalar decays to $γγ$, $Zγ$, and $Wγ$ in the Georgi-Machacek model

We compute the decay widths for the neutral and singly-charged Higgs bosons in the Georgi-Machacek model into the final states $γγ$, $Zγ$, and $Wγ$. These decays are most phenomenologically interesting for the fermiophobic custodial fiveplet states $H_5^0$ and $H_5^{\pm}$ when their masses are below threshold for decays into $WW$, $ZZ$, or $WZ$. We study the allowed branching ratios into these final states using scans over the allowed parameter space, and show how the model can be constrained by LEP searches for a fermiophobic Higgs boson decaying to two photons. The calculation involves evaluating one-loop diagrams in which the loop contains particles with two different masses, some of which do not appear in the existing literature. We give results for these diagrams in a form convenient for numerical implementation using the LoopTools package.

hep-ph↗

arXiv2017

Characterizing a benchmark scenario for heavy Higgs boson searches in the Georgi-Machacek model

The Georgi-Machacek model is used to motivate and interpret LHC searches for doubly- and singly-charged Higgs bosons decaying into vector boson pairs. In this paper we study the constraints on and phenomenology of the "H5plane" benchmark scenario in the Georgi-Machacek model, which has been proposed for use in these searches. We show that the entire H5plane benchmark is compatible with the LHC measurements of the 125 GeV Higgs boson couplings. We also point out that, over much of the H5plane benchmark, the lineshapes of the two CP-even neutral heavy Higgs bosons $H$ and $H_5^0$ will overlap and interfere when produced in vector boson fusion with decays to $W^+W^-$ or $ZZ$. Finally we compute the decay branching ratios of the additional heavy Higgs bosons within the H5plane benchmark to facilitate the development of search strategies for these additional particles.

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